Hemoglobin is a conjugated protein that is present in all red blood cells. Red blood cells have a concave, disc shape with an approximately 10 micrometer diameter, and they commonly exist in blood vessels at densities of about five million red blood cells per cubic millimeter of blood. Also, it is well known that red blood cells both scatter and transmit light incident thereon in amounts that vary as a function of the oxygen content of the hemoglobin in the cells. The differential between absorption of light by oxygenated and non-oxygenated hemoglobin as light is transmitted through red blood cells provides a convenient basis for measuring the oxygen saturation level of the blood flowing in a blood vessel.
In the prior art, optical fiber sensors have been developed to detect and measure the amount of oxygenated hemoglobin present in the bloodstream in relation to all of the hemoglobin present in the red blood flowing through a blood vessel. The oxygen saturation level measurement is performed by utilizing an intravascular catheter that includes transmitting and receiving optical fibers for respectively conducting light to and returning light from an in vivo measurement site. The distal end of the transmitting optical fiber is commonly oriented in a co-planar relationship with the distal end of the receiving optical fiber, at the distal end of the catheter.
Light is both absorbed and back scattered by the red blood cells in the vicinity of the in vivo measurement site, with the amount of absorption varying as a function of the oxygen content of the red blood cell hemoglobin. A portion of the back scattered light enters the receiving optical fiber and is directed to an external photodetector that measures the intensity of the back scattered light. Due to the variation in radiation absorption caused by changes in the oxygen saturation of the hemoglobin in the red blood cells, the total amount of back-scattered radiant energy at the photodetector varies as a function of this oxygen saturation. For a complete description of the use of such an optical fiber sensor in an application for measurement of oxygen saturation in red blood cells, see U.S. Pat. No. 4,623,248 (Sperinde et al.), which is assigned to the assignee of the present invention.
One prospective application of an optical fiber sensor of the type described above would be to measure the continuous jugular venous oxygen saturation (SjvO2) of the blood flowing from the brain. If a cerebral (head) injury has reduced the amount of blood flowing into the skull, the brain compensates for the reduced blood flow by absorbing a greater amount of oxygen (reducing the percentage of oxygen saturation) from the available blood. Thus, the measurement of SjvO2 provides an excellent indicator of cerebral hypoxia/ischemia (reduced blood flow to the head), because it is directly linked to the amount of oxygen consumed by the brain from the available flow of blood into the skull.
In the prior art, a relatively small diameter catheter with an optical fiber sensor has been employed in attempts to measure the SjvO2 of blood exiting the head of a patient. Typically, a SjvO2 monitor is coupled to the proximal ends of a pair of optical fibers that extend longitudinally along the length of the catheter. In this prior art system, the distal ends of the optical fibers are disposed along the periphery and at the distal end of the catheter, which is adapted for disposition at a desired point in a blood vessel (vein). However, the accuracy with which SjvO2 has been measured with the prior art sensor has been disappointing. The accuracy of the sensor was found to be inconsistent, since it would at times yield measurements with unacceptable error. It was not clear what the source of the inconsistency and error could be, since the same sensor and catheter was found to provide acceptable results when used to monitor oxygen content in blood flowing through the heart. A discovery of the cause of this problem has led to the present invention.
To partially compensate for the problem, users have employed in vivo calibration of SjvO2 optical fiber sensors. Generally, the calibration is performed as follows: (1) the medical practitioner draws a blood sample through a lumen of a catheter disposed in a blood vessel; (2) a laboratory immediately measures the SjvO2 of the blood sample; (3) the medical practitioner determines the difference or offset between the laboratory's measured SjvO2 value and the SjvO2 value indicated by the optical fiber sensor; and (4) the medical practitioner compensates (increases or decreases) the SjvO2 value indicated by the optical fiber sensor with the offset determined by the calibration. However, since this compensated measurement of SjvO2 is not exact and the accuracy of the prior art SjvO2 sensor has been found to vary over time, frequent laboratory tests must be performed to verify the true SjvO2 value. Additionally, since in vivo calibration is only accurate for a specific location, the calibration must be repeated every time the disposition of the catheter is changed.
Therefore, there is a need for a catheter mounted O2 sensor to solve the problem of providing continuous and accurate percentages of the oxygen saturation of hemoglobin for blood flowing through a small diameter blood vessel, such as the jugular vein. The prior art approach of frequently calibrating such a sensor in vivo clearly does not satisfy this need.